This invention relates generally to processes and systems for removing hydrogen sulfide from a gaseous stream. More specifically the invention relates to improvements in a known process and system wherein hydrogen sulfide is removed from a gaseous stream, using as an oxidizing agent a nonaqueous scrubbing liquor in which are dissolved sulfur and a reaction-promoting base.
In the present inventor""s U.S. Pat. Nos. 5,733,516 and 5,738,834 (the entire disclosures of which are hereby incorporated by reference), a process and system are disclosed which use a sulfur-amine nonaqueous sorbent (SANS) for removal of hydrogen sulfide (H2S) from gas streams. Pursuant to the said process, a sour gas stream containing H2S is contacted with a nonaqueous sorbing liquor which comprises an organic solvent for elemental sulfur, dissolved elemental sulfur, an organic base which drives the reaction converting H2S sorbed by the liquor to a nonvolatile polysulfide which is soluble in the sorbing liquor, and an organic solubilizing agent which prevents the formation of polysulfide oil-which can tend to separate into a separate viscous liquid layer if allowed to form. The sorbing liquor is preferably water insoluble as this offers advantages where water soluble salts are desired to be removed. Hydrogen sulfide (H2S) gas is sorbed into this sorbing liquor where it reacts with the dissolved sulfur in the presence of the base to form polysulfide molecules. This reaction decreases the equilibrium vapor pressure of H2S over the solution, thus providing more efficient scrubbing than a physical solvent. The liquor is then sent to a reactor where sufficient residence time is provided to allow the polysulfide forming reactions to reach the desired degree of completionxe2x80x94i.e., resulting in a nonvolatile polysulfide which is soluble in the sorbing liquor. From the reactor, the liquor flows to a regenerator where the solution is oxidized (e.g., by contact with air), forming dissolved elemental sulfur and water (which, being insoluble, is rejected from the solution either as an insoluble liquid layer or as water vapor exiting the overhead of the regenerator or absorber). The temperature of the liquor, which up to this point is sufficient to maintain the sulfur in solution, is then lowered, forming sulfur crystals, which are easily removed by gravity settling, filtration, centrifuge, or other standard removal method. Enough sulfur remains dissolved in the liquor following separation of the sulfur crystals that when this solution is reheated and returned to the absorber for recycling in the process, a sufficient amount of sulfur is present to react with the inlet H2S gas.
The process and system for removal of hydrogen sulfide from a gaseous stream in accordance with my U.S. Pat. Nos. 5,733,516 and 5,738,834 patents thus utilize a nonaqueous sorbent liquor comprising a solvent having a high solubility for elemental sulfur, and a sufficient temperature so that solid sulfur formation does not occur either in the hydrogen sulfide absorber or in the air-sparged regenerator of the system utilized for carrying out the process. The solvent generally can have a solubility for sulfur in the range of from about 0.05 to 2.5, and in some instances as high as 3.0 g-moles of sulfur per liter of solution. The temperature of the nonaqueous solvent material is preferably in the range of about 15.degree. C. to 70.degree. C. Sulfur formation is obtained, when desired, by cooling the liquor proceeding from the air-sparged regenerator. This can for example be effected at a sulfur recovery station by cooling means present at the station. The solvent is thereby cooled to a sufficiently low temperature to crystallize enough solid sulfur to balance the amount of hydrogen sulfide absorbed in the absorber. The solubility of elemental sulfur increases with increasing temperature in many organic solvents. The rate of change of solubility with temperature is similar for many solvents, but the absolute solubility of sulfur varies greatly from solvent to solvent. The temperature change necessary to operate the process will vary primarily with the composition of the sorbent the flow rate of sorbent, and the operating characteristics of the recovery station. For most applications, a temperature difference of 5.degree. C. to 20.degree. C. is appropriate as between the temperature of the solvent material at the absorber/reactor and temperature to which the said solvent is cooled at the sulfur recovery station; but the temperature difference can in some instances be as little as 3 .degree. C. or as much as 50 .degree. C. The nonaqueous solvent comprises a solvent selected from the group consisting of 1, 2, 3, 4 tetrahydronaphthalene, N, N dimethylaniline, diphenyl ether, dibenzyl ether, terphenyls, diphenylethanes, alkylated polycyclic aromatics, and mixtures thereof.
In order to obtain a measurable conversion of sulfur and hydrogen sulfide to polysulfides, the base added to the solvent must be sufficiently strong and have sufficient concentration to drive the reaction of sulfur and hydrogen sulfide to form polysulfides. Most primary, secondary and tertiary amines are suitable bases. More particularly, amines which comprise nitrogen connected to alkane groups, alkanol groups, benzyl groups, or hydrogen (but not to phenyl) are suitable. It should be noted that while the solvent utilized requires the addition of a base to promote the reaction of sulfur and hydrogen sulfide to form polysulfides, the base and the solvent may be the same compound.
The base may be a tertiary amine. Polysulfide compounds formed in the presence of tertiary amines are much more easily converted to sulfur by air during the regeneration step than those formed from primary amines or secondary amines. The base is more preferably selected from the group consisting of 2-(dibutylamino) ethanol, N-methyldicyclohexylamine, N-methyl-diethanolamine, tributylamine, dodecyldimethylamine, tetradecyldimethylamine, hexa-decyldimethylamine, diphenylguanidine, alkylaryl polyether alcohols, and mixtures thereof. The base is present at concentrations of about 0.01M to 2.0M. Of the bases cited, 2-(dibutylamino) ethanol and N-methyldicyclohexylamine are most preferred, and are preferably present at concentrations of about 0.5 to 1.0 M.
The nonaqueous sorbing liquor, in addition to including a solvent having a high solubility for sulfur, and a base, comprises an agent suitable for maintaining the solubility of polysulfide intermediates which may otherwise separate when they are formed during operation of the process. Such solubilizing agent is preferably selected from the group consisting of benzyl alcohol, benzhydrol, 3-phenyl-1-propanol, tri(ethylene glycol), and mixtures thereof.
During operation of this SANS process, most of the H2S removed from the gas stream is converted to elemental sulfur. However, a small fraction of the absorbed H2S is oxidized further to thiosulfate and sulfate species, which are soluble in the sorbent (probably as the salt of the protonated amine). Unrestricted buildup of the sulfur oxyanion species can eventually cause an increase in the viscosity of the solution and reduce absorption rates. In its protonated form, the amine is not an effective catalyst for the absorption of H2S and subsequent conversion to polysulfidc. Therefore, the regeneration process must liberate the free amine from its protonated form as well as remove the sulfate and thiosulfate. Byproduct salt buildup can also result in uncontrolled deposition of sulfate or thiosulfate salts in the system. Therefore, some means of efficient removal of thiosulfate and sulfate from the circulating solution must be employed.
Washing a portion of the sorbent with water or a mildly alkaline solution is disclosed in the U.S. Pat. Nos. 5,733,516 and 5,738,834 patents, and is a workable method. In this case, the sulfate and thiosulfate are extracted into an aqueous phase where they are more soluble. Making the aqueous phase alkaline retards the transfer of amine from the nonaqueous phase if the xe2x80x9cfree basexe2x80x9d form of the amine is water insoluble. However, such a washing step has several disadvantages. In practice, it is difficult to avoid emulsion formation and transport of liquid water throughout the system. In addition, the need for amines and other solvent components to be water-insoluble is an undesirable restriction on selection of these components. Furthermore, large amounts of water may have a deleterious effect on the SANS process operation.
Now in accordance with the present invention, it has been found that the foregoing problems of the prior art SANS process can be overcome by use of a new method for removing thiosulfate and sulfate from the SANS process solution. This method is based on addition of gaseous ammonia to the process solution. Surprisingly, it has been found that the ammonium salts of thiosulfate and sulfate are quite insoluble in the nonaqueous SANS solution, in contrast to their high solubility in aqueous solutions. Therefore, bubbling ammonia into a SANS solution containing these salts results in nearly instantaneous formation of solid ammonium sulfate and ammonium thiosulfate which precipitate from the solution, thereby allowing their removal by settling, filtration, or other common solid/liquid separation methods. The reaction (for sulfate) can be written as follows, with B representing the amine (HB30 is then the protonated amine):
(HB)2SO4+2NH3xe2x86x92(NH4)2SO4↓+2B
Since no water is used, this method avoids the problem of emulsion formation and transport of water around the system. In addition, the reactions appear to be essentially quantitative, and since ammonia is quite inexpensive, the removal process is economically favorable. Water soluble components can be used in the sorbent without losing them to the water wash or using other separation steps to recover them from the wash water of the earlier method.
Removal of the sulfate and thiosulfate with ammonia causes a decrease in the electrical conductivity of the solution. Thus measurement of the solution conductivity can provide a means of monitoring the status of byproduct removal and thus a means of controlling the rate of addition of ammonia.